Many uncoated papers present a so called watermark, for instance in the form of the manufacturer's name or in the form of some distinctive symbol. The watermark is normally incorporated by commensurate patterning of a wire in the paper machine, for instance the wire located on the dandy roll. The watermark in the paper can be clearly seen when the paper is directed towards a light source, for instance towards a window of a room. This type of paper is one form of security paper.
The need for surface treated paper, primarily coated paper, increases from year to year. When a base paper which incorporates a watermark is coated, the watermark will be covered and hidden by the coating layer, so that the paper can no longer be easily recognized.
Many ways of marking finished paper so that something resembling watermarks appears in the paper are known to the art.
According to the international (PCT) application WO 94/07694 a chemical is applied to the paper such as to render the paper transparent over those areas in which the chemical has been applied. The chemical is applied by transferring a solution of the chemical in question via a number of rolls to a stamp means mounted on a roll that is located in the immediate proximity of another roll, and by passing the paper through the nip defined between said two rolls and therewith transfer the chemical solution from the stamp means to the paper. This chemical paper watermarking process is carried out conveniently in a printing press, either immediately before or immediately after the actual printing process.
According to the German patent specification 37 18 452 a plurality of marks are embossed mechanically in the paper and it is stated that the paper becomes transparent at the embossed locations. This process is achieved with a roll that includes a plurality of patterned elevated surfaces. A corresponding number of anvil surfaces are provided on a shaft which is spaced slightly from said elevations, said anvil surfaces having a breadth which covers the elevations on the embossment roll. The paper is introduced into the gap between the elevations and the anvil surfaces and because the gap is slightly smaller than the thickness of the paper, the pattern defined by the elevated embossments is embodied in the paper.